Technical Problem
In connection with dyeing cellulose fiber material by means of reactive dyeing agents according to the exhaust method there is often a risk that uneven dyeing will result, especially when using dyeing agents with a high reactivity, once the alkali necessary for the reaction is added to the fibers, the fixation starts with a high velocity and reaches high values within a short time interval. If one plots the fixation values for such a dyeing procedure in percent of the final fixation relative to the dyeing time, one obtains a steep fixation characteristic, at which an unproportionally large part of the added reactive dyeing agent will be fixed during an unproportionally short part of the dyeing time interval. As an example the design paper Levafix-Brillantgelb E-GA of the design schedule LeH1350 from Bayer AG can be mentioned.
In order to secure a high degree of evenness during difficult dyeing conditions, it is necessary to reduce the steep increase of fixation in the initial phase of the dyeing process and to influence the reaction with fibers in such a way, that the fixation proceeds at least approximately proportionally relative to the dyeing time.
This can for example be achieved by adding the alkali not at the optimal dyeing temperature for the reactive dyeing agent in question, but at a lowest possible initial temperature, for example 20.degree. C., then warming slowly up to the dyeing temperature. As the reaction velocity is dependent on the temperature, the fixation will initially be held back, so that the desired more level fixation characteristic can be obtained.
However, this operation has serious disadvantages. In several cases one cannot obtain a sufficiently low initial temperature, as for example in tropical countries, where the temperature of the available water for operation can be considerably higher. In cases of the today widely spread warm dyeing reactive dyeing agents the recommended operation temperature of which lies at 40.degree.-50.degree. C. the measure described is no longer effective, as sufficient temperature difference between initial and dyeing temperature is no longer obtained. Furthermore, low treatment temperatures in many ways counteract the good quality which is desired. For example, various goods made in jet dyeing machines and in winch dyeing machines open in a poor way and tend to be displaced. In cheese dyeing machines the flow through of the bath is prevented when operating at lower temperatures.
However, if one, in order to eliminate said disadvantages, adds the alkali at the optimal dyeing temperature, this must absolutely proceed in portions, as otherwise--as described initially--a too steep fixation characteristic is obtained. These additions must be calculated very carefully and proceed in controlled time intervals in order to avoid sudden fixation jumps, which again result in an uneven dyeing. Such an operation prolongs the dyeing time and reduces the production capacity. Furthermore, this manner of operation calls for increased personnel and involves risk of incorrect operation.
In order to avoid the hitherto known disadvantages further proposals have been made known by means of which the desired evenness, level dyeing will be achieved. In the EP 0-017-805 U.S. Pat. No. 4,372,744 to Hildebrand et al; assigned to Bayer AG it has been proposed to introduce the reactive dyeing agent into the bath, containing electrolyte and alkali in such a way, that less than 10% of the dyeing agent fixed at the end of the dyeing process are present unfixed in reactive form during the dye bath. However, it has not been possible before to introduce this method into the dyeing practice, which partly depends on the fact that continuous measurement/testing of the concentration of the reactive dyeing agent in the dye bath involves extremely high costs (HPLC or by separate serial tests).
An isotermal dyeing method is also known and in which to a bath containing a dyeing agent and electrolyte at dyeing temperature the alkali necessary for the fixation of the reactive dyeing agent has been added automatically and progressively. (EP 0 126 042 which corresponds to U.S. Pat. No. 4,562,604). For realizing this method a computer controlled dosage apparatus is required, which substantially continuously adds the alkali over a predetermined time interval so, that in an initial phase the amount of said alkali is of a small quantity, which during the process is increased progressively until the total quantity of alkali is added.
According to EP 0 126 042 the addition of the dosage process can for example be achieved if the addition of alkali follows an exponential function. By a mathematic mixture with a linear function, different increments can be obtained. (FIG. 4 of EP 0 126 042). The method claimed in EP 0 126 042 has already been positively accepted in the dyeing practice. However, it has appeared that besides all these advantages it has certain disadvantages. In FIG. 1 of the appended drawing a typical fixation characteristic for a sulphatoethylsulphon dyeing agent is shown. However, the characteristic shows a substantially linear extension of the main part of the fixation. However, the curved part of the characteristic at the beginning of the addition and the levelling part of the characteristic at the end of the dyeing time results in an unnecessary prolongation of the dyeing time (compare Ungerman, Textil Praxis International 39 (1984), page 495, FIG. 8). Consequently an unpredictable deviation from the ideal process of the characteristic result occurs. This is shown in FIG. 1 by means of dashed lines. If one follows the previous fixation process one might by means of a change of the beginning and the end of the characteristic addition achieve obvious time saving. Laying the previous need for time as a basis, a clearly more level fixation characteristic mode will be obtained upon an ideal use of this time period and this would result in an improved eveness and/or an improved operation safety. A fixation mode according to FIG. 1 is achieved upon optimal choice of addition increments.
If one adds alkali at initially greater increments, the dyeing time will be reduced within certain limits, however, this will be paid for with undesired more steep fixation process. Then the risk for an uneven dyeing will be considerably increased (compare also Ungermann, Textil Praxis International Vol. 39 (1984) page 495, FIG. 6).
Also upon a reduction of the time for addition the dyeing time will be reduced only upon payment for a more steep process of the characteristic, as the characteristic for the addition will be the same.
From what is said above, it is apparent that when using the previously known and in EP 0 126 042 described progressive addition technique the fixation can apparently not be influenced in such a way that the fixation process follows the ideal mode or follows the same in a better way.